1. Field of the Invention
The present invention relates to an original reader having an illumination unit comprised of a plurality of light emitter arrays each formed by a plurality of light emitters arranged in a row.
2. Description of the Related Art
Conventionally, an illuminating light source used in an original reader has often been implemented by an Xe fluorescent tube filled with Xe (xenon) gas or the like.
However, a fluorescent tube, such as an Xe tube, which is driven for emission of light by high-frequency alternating current, needs an inverter for converting a DC voltage into an AC voltage, and hence it is required to secure space for disposing the inverter in the original reader. Further, since a fluorescent tube such as an Xe tube has a low light-emitting efficiency, a large amount of heat is generated from the inverter and the fluorescent tube. Therefore, countermeasures against the heat generation and corresponding electric power are required. In addition, the service life of a fluorescent tube, such as an Xe tube, is relatively short due to light amount deterioration caused by aging.
On the other hand, an LED (light emitting diode) which is capable of emitting an appropriate amount of light has been developed so as to solve the problems from which an Xe tube or the like fluorescent tube suffers, and there has been proposed an original reader provided with an illuminating light source formed by arranging a plurality of LEDs (see Japanese Paten Laid-Open Publications (Kokai) No. 2002-247296 and No. 2005-051381).
In the original reader, as shown in FIG. 7, e.g. 48 white LEDs 201 are linearly arranged on an LED circuit board 200 in the main scanning direction of an original to form the illuminating light source.
In this case, the 48 LEDs 201 connected in series are divided into six blocks 301, as shown in FIG. 8, and each of the blocks 301 is connected to a driving circuit 302. The driving circuit 302 is a constant current circuit. Assuming that a forward voltage per one LED 201 is 3.3 V, the driving circuit 302 applies a voltage higher than 3.3 V×6=19.8 V to thereby feed a desired electric current to each block 301.
In principle, the 48 LEDs 201 may be all connected in series to form a constant current circuit. However, this circuit requires more than 158.4 V (=3.3 V×48) as a voltage to be supplied, and hence is not practical. For this reason, there is used such a simple constant current circuit as shown in FIG. 8. In this case, the amount of light emitted from the LEDs 201 can slightly differ from block to block, but this does not matter because the differences can be sufficiently corrected by shading correction, referred to hereinafter.
In the illustrated example, it is assumed that the driving circuit 302 applies a voltage of 24 V to the block 301 to thereby feed a standard constant current of 20 mA. Since the driving circuit 302 is a constant current circuit, a stable current of 20 mA flows, which stabilizes the light amount of each LED 201, so that the LEDs 201 functions as an excellent illuminating light source of the original reader.
In the above-described prior art, however, the 48 LEDs 201 connected in series are divided into six blocks 301. For this reason, if one LED 201 is disconnected or faulty or if one driving circuit 302 fails, all including normal LEDs 201 in the block 301 cannot be turned on.
As a consequence, the original reader is incapable of performing an image reading operation until a part of the driving circuit 302 or the LED 201 in trouble is replaced.